Exemplary embodiments of the present invention relate to an illumination device and a method for illuminating a target using this illumination device.
Multiple active laser target illumination devices and also laser target illumination methods are known from the prior art, in which a target object is illuminated by laser radiation. In particular, the range gating method is known, in which pulsed laser radiation is used to illuminate a target surface, so that suitable cameras can register the reflected radiation of this illumination. Due to the pulsing of the laser radiation it is possible to determine a distance to the target surface. Furthermore, the laser target illumination is used in the prior art in active optical tracking methods of rapidly moving targets.
However, as a result of the coherence of the laser radiation typically used in laser target illumination and the effect of the atmospheric turbulence occurring on the beam path and also the non-ideal flat surface structure of the target, interference-related disturbances can occur, which are also called speckle effects. These speckle effects arise due to strong local intensity variations of the reflected light, which strongly decreases the image quality. Such a decreased image quality makes subsequent electronic analysis more difficult and often makes an optical tracking method impossible.
Exemplary embodiments of the present invention are directed to an illumination device that allows a high image quality of the illuminated objects with simple and cost-effective production and assembly and with reliable and low-maintenance operation. Exemplary embodiments of the present invention are also directed to a method for illuminating a target that allows the recording of high-quality images of an illuminated object with simple and cost-effective application.
Exemplary embodiments of the present invention thus provide an illumination device suitable for illuminating a target to be combated. The illumination device according to the invention comprises a light source, which is preferably a laser light source. The light source is configured to emit light along a beam path. Furthermore, a first modulator is introduced into the beam path, wherein the modulator is configured to manipulate a direction of the beam path in a first plane. It is therefore preferably provided that the modulator can deflect the light radiation originating from the light source by a predefined angle. In particular, it is provided that the first modulator is an active optical element. In addition, a phase plate is provided in the beam path, which has a variety of optical thicknesses. The phase plate is preferably arranged downstream of the first modulator, so that the light of the light source can initially be deflected by the modulator before it is incident on the phase plate. The variety of different optical thicknesses of the phase plate can preferably be implemented in that the phase plate has an uneven surface. Therefore, preferably statistically distributed optical path differences of the light arise within the beam path. It is particularly preferred that these optical path differences on average make up small fractions of the wavelength of the light emitted by the light source. A control device is connected to the light source and the first modulator, so that the control device can activate the first modulator using different frequencies. In this manner, the beam path can be deflected over a first control period of time up to a first angle. The extent of the first duration and the first angle can be individually established by the control device. However, it is particularly preferably provided that the first angle is at most 5 millirad, while the control period of time is in the range from 1 to 10 ns (inclusive).
Preferably the light source emits light pulses. It is particularly preferred that a light pulse has the duration of the control period of time, so that the first modulator can be activated synchronously to the emission of light pulses. In this manner, it is advantageously possible to deflect each of the emitted light pulses up to a predetermined angle. A broad scattering of the illumination is therefore possible. Alternatively, it is also particularly preferred that the light source emits multiple light pulses during the control period of time. In any case, it is particularly preferred that at least one complete light pulse is emitted during the control period of time. It is therefore preferably ensured that each light pulse can be deflected completely by the first modulator.
Furthermore, it is preferable that the first modulator is an acousto-optical modulator. The first modulator is particularly preferably a Bragg cell. Therefore, the first modulator comprises a transparent carrier material, through which a wave, in particular an acoustic wave, is conducted. Different optical densities arise within the carrier material due to the acoustic wave, so that a diffraction of the beam path can be executed on an optical lattice resulting from the different optical densities. The beam path can be manipulated in this manner. The degree of the deflection can be varied by the activation of the first modulator using various frequencies, which means the emission of different sound waves. It is therefore preferably possible to design the manipulation of the beam path to be greatly variable. The first modulator is particularly preferably activated using frequencies of at least 100 MHz.
The difference of the optical thicknesses and the beam plate advantageously has a value that does not fall below 1 μm, in particular 5 μm. Alternatively or additionally, it is preferably provided that the value of the difference of the variety of the optical thicknesses of the beam plate does not exceed 50 μm, particularly preferably 10 μm. Therefore, the beam path deflected by the first modulator can pass various optical thicknesses of the beam plate, wherein statistically distributed optical path differences arise due to the various optical thicknesses. It is nonetheless ensured by the above-mentioned values of the differences of the optical thicknesses that these path differences are on average small fractions of the laser wavelength, which represents optimum conditions for effective target illumination.
In an advantageous embodiment of the invention, the illumination system comprises a second modulator, which is implemented as identical to the first modulator. The second modulator is preferably configured to manipulate a direction of the beam path in a second plane. The second plane is arranged in particular perpendicularly to the first plane. The use of the second modulator therefore advantageously allows the beam path to be deflected in all spatial directions. The variety of the possible deflection directions can therefore be increased, which allows an effective variation of the path differences of the light.
Furthermore, the illumination device preferably has an optical sensor, which can be a camera. The optical sensor is preferably configured to detect the light emitted by the light source and reflected from the target. Since the light source emits light pulses, it is furthermore particularly preferable that a determination of a distance of the target to the illumination device is possible by means of the detection of the reflected light pulses by the optical sensor.
The illumination device according to the invention as described above allows an effective reduction of the speckle effect described above. The beam path is conducted through various optical thicknesses of the beam plate by the deflection of the beam path by means of the first modulator. Therefore, as already described, statistically distributed optical path differences arise. In this manner, it is possible to generate local and chronological variations of the phase fronts of the emitted light pulses in the cross-section of the beam path. Interactions with atmospheric disturbances or a non-ideal flat target surface can therefore result in chronologically and locally varying speckle effects. It is possible by way of a time averaging of these local and chronological variations to reduce the influence of the speckle effect on the overall measurement.
Furthermore, the illumination device according to the invention or according to a preferred refinement of the invention allows an illumination of the target in which an intensity profile over the beam path is similar to a plateau. In contrast thereto, in conventional illumination devices, a Gaussian distribution would occur, wherein the intensity decreases toward the edge of the beam path. Therefore, the illumination device according to the invention allows uniform and therefore optimized illumination of the target.
The invention furthermore relates to a method for illuminating a target, wherein an illumination device having the above-described features is used. According to the invention, the following steps are executed in the specified sequence: First a light pulse is emitted by the light source. Subsequently, the first modulator and/or the second modulator is activated in such a manner that the light pulse just emitted is deflected in a randomly determined direction. The light pulse therefore runs through a randomly determined region of the phase plate, so that statistically distributed optical path differences arise over the cross-section of the beam path. The light pulse thus manipulated is preferably oriented onto the target, so that it can illuminate the target.
In one preferred embodiment, the above-mentioned steps are executed in continuous repetition. It is therefore particularly preferably provided that the light source continuously emits light pulses, wherein the light pulses are deflected in randomly determined directions. Therefore, the above-described chronological and spatial variation of the phase front distribution results over the cross-section of the beam path in the different emitted light pulses.
Furthermore, it is preferably provided that the additional following steps are executed: First, each emitted light pulse is received, which is performed in particular by the optical sensor. Subsequently, the intensities of the received light pulses are determined, wherein preferably the phase front distribution over the cross-section of the reflected beam path can also be determined. A chronological mean value of the intensity of each pulse is determined. In this manner, it is possible to reduce the influence of speckle effects, since the speckle effect is spatially and chronologically varied by way of the above-described emission of the light pulses and manipulation of the direction of the emitted light pulses. Finally, it is preferable that an image is generated from the averaged intensity data. This image allows in particular an optical tracking method to be carried out, using which a rapidly moving target can be tracked by an active unit, for example.